Resistance variable memory structure and method of forming the same

ABSTRACT

A semiconductor structure includes a resistance variable memory structure. The semiconductor structure also includes a dielectric layer. The resistance variable memory structure is over the dielectric layer. The resistance variable memory structure includes a first electrode disposed over the dielectric layer. The first electrode has a sidewall surface. A resistance variable layer has a first portion which is disposed over the sidewall surface of the first electrode and a second portion which extends from the first portion away from the first electrode. A second electrode is over the resistance variable layer.

TECHNICAL FIELD

This disclosure relates generally to a semiconductor structure and, more particularly, to a resistance variable memory structure and method of forming a resistance variable memory structure.

BACKGROUND

In integrated circuit (IC) devices, resistive random access memory (RRAM) is an emerging technology for next generation non-volatile memory devices. RRAM is a memory structure including an array of RRAM cells each stores a bit of data using resistance, rather than electronic charge. Particularly, each RRAM cell includes a resistance variable layer, the resistance of which can be adjusted to represent logic “0” or logic “1”.

From an application point of view, RRAM has many advantages. RRAM has a simple cell structure and CMOS logic comparable processes which result in a reduction of the manufacturing complexity and cost in comparison with other non-volatile memory structures. Despite the attractive properties noted above, a number of challenges exist in connection with developing RRAM. Various techniques directed at configurations and materials of these RRAMs have been implemented to try and further improve device performance.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure may be understood from the following detailed description and the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a flowchart of a method of forming a semiconductor structure having a resistance variable memory structure according to at least one embodiment of this disclosure.

FIGS. 2A to 2F are cross-sectional views of semiconductor structures having a resistance variable memory structure at various stages of manufacture according to one or more embodiments of the method of FIG. 1.

FIG. 2G is a planar view of the semiconductor structure having the resistance variable memory structure of FIG. 2F.

FIG. 2H is a cross-sectional view taken along line A-A′ in FIG. 2G to show the semiconductor structure in operation with filaments formed in the resistance variable layer according to one or more embodiments of this disclosure.

FIG. 3 is a cross-sectional view of a semiconductor structure having the resistance variable memory structure in FIG. 2D (or 2E) according to at least one embodiment of this disclosure.

FIG. 4 is a cross-sectional view of a semiconductor structure having the resistance variable memory structure in FIG. 2D (or 2E) according to some embodiments of this disclosure.

DETAILED DESCRIPTION

The making and using of illustrative embodiments are discussed in detail below. It should be appreciated, however, that the disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative and do not limit the scope of the disclosure.

According to one or more embodiments of this disclosure, a semiconductor structure includes a resistance variable memory structure. The resistance variable memory structure includes a resistance variable layer formed between two electrodes. By applying a specific voltage to each of the two electrodes, an electric resistance of the resistance variable layer is altered. The low and high resistances are utilized to indicate a digital signal “1” or “0”, thereby allowing for data storage. The switching behavior depends not only on the materials of the resistance variable layer but also on the choice of electrodes and interfacial properties of the electrodes.

According to one or more embodiments of this disclosure, the semiconductor structure having a resistance variable memory structure is formed within a chip region of a substrate. A plurality of semiconductor chip regions is marked on the substrate by scribe lines between the chip regions. The substrate will go through a variety of cleaning, layering, patterning, etching and doping steps to form the semiconductor structures. The term “substrate” herein generally refers to a bulk substrate on which various layers and device structures are formed. In some embodiments, the bulk substrate includes silicon or a compound semiconductor, such as GaAs, InP, Si/Ge, or SiC. Examples of the layers include dielectric layers, doped layers, polysilicon layers or conductive layers. Examples of the device structures include transistors, resistors, and/or capacitors, which may be interconnected through an interconnect layer to additional integrated circuits.

FIG. 1 is a flowchart of a method 100 of forming a semiconductor structure having a resistance variable memory structure according to at least one embodiment of this disclosure. FIGS. 2A to 2F are cross-sectional views of a semiconductor structure 200 having a resistance variable memory structure at various stages of manufacture according to various embodiments of the method 100 of FIG. 1. Additional processes may be performed before, during, or after the method 100 of FIG. 1. Various figures have been simplified for a better understanding of the inventive concepts of the present disclosure.

Referring now to FIG. 1, the flowchart of the method 100 begins with operation 101. In at least one embodiment, a dielectric layer is formed over a substrate. At least one conductive structure is formed over the substrate and embedded in the dielectric layer. The at least one conductive structure has a portion exposed to a top surface of the dielectric layer. A first electrode material is deposited over the conductive structure and the dielectric layer.

Referring to FIG. 2A, which is a cross-sectional view of a portion of a semiconductor structure 200 having a resistance variable memory structure after performing operation 101. The semiconductor structure 200 includes a substrate (not shown). In the illustrated examples of FIGS. 2A-2F, the semiconductor structures 200 include a dielectric layer 201 formed on a top surface of the substrate (not shown). In at least one embodiment, the dielectric layer 201 includes one or more dielectric layers. The dielectric layer 201 comprises silicon oxide, fluorinated silica glass (FSG), carbon doped silicon oxide, silicon nitride, silicon oxynitride, tetra-ethyl-ortho-silicate (TEOS) oxide, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), BLACK DIAMOND® (Applied Materials of Santa Clara, Calif.), amorphous fluorinated carbon, low dielectric constant (low-k) dielectric material, or combinations thereof. The deposition process may include chemical vapor deposition (CVD), atomic layer deposition (ALD), high density plasma CVD (HDPCVD) or spinning on glass.

A conductive structure 203 is formed embedded in the dielectric layer 201. In certain embodiments, the conductive structure 203 includes a conductive interconnect, a doped region or a silicide region. In some embodiments, the conductive structure 203 includes aluminum, aluminum alloy, copper, copper alloy, titanium, titanium nitride, tantalum, tantalum nitride, tungsten, metal silicide, silicon or combinations thereof. In the illustrated example of FIG. 2A, the semiconductor structure 200 may be formed by lithography patterning and etching in the dielectric layer 201. A layer of the conductive structure 203 is deposited over the patterned dielectric layer 201 and subsequently planarized to form the conductive structure 203. A top surface 203A of the conductive structure 203 is substantially coplanar with a top surface 201A the dielectric layer 201.

A first electrode material 205 is deposited over top surfaces (203A and 201A) of the conductive structure 203 and the dielectric layer 201. The first electrode material 205 includes a conductive material having a proper work function such that a high work function wall is built between the first electrode material 205 and a resistance variable layer subsequently formed. The first electrode material 205 may comprise Pt, AlCu, TiN, Au, Ti, Ta, TaN, W, WN, Cu or combinations thereof. Possible formation methods include electroless plating, sputtering, electro plating, PVD or ALD. In some embodiments, the first electrode material 205 is electrically connected to an underlying electrical component, such as a transistor, through the conductive structure 203.

Referring back to FIG. 1, method 100 continues with operation 102. In operation 102, the first electrode material is patterned to form a first electrode. The first electrode has a top surface and a sidewall surface.

Referring to FIG. 2B, which is a cross-sectional view of a portion of the semiconductor structure 200 after performing operation 102. A mask layer 207 having a feature is formed over the first electrode material 205 and also over the conductive structure 203. The feature is formed by a suitable process, including deposition, lithography patterning, and/or etching processes. An etching process is performed to remove the first electrode material 205 not underlying the feature of the mask layer 207. Then, a first electrode 205E is formed and contacts the conductive structure 203.

The mask layer 207 is removed after the etching process from the semiconductor structure 200 and a top surface 205A of the first electrode 205E is exposed. Also, the first electrode 205E has a sidewall surface 205B connected to the top surface 205A. The removing process of the mask layer 207 may include a dry etching process, wet etching process, or combination thereof.

Referring back to FIG. 1, method 100 continues with operation 103. In operation 103, a resistance variable layer and a second electrode material are deposited over the top surface and the sidewall surface of the first electrode.

FIG. 2C is a cross-sectional view of the semiconductor structure 200 after performing operation 103. A resistance variable layer 209 is conformally deposited over the top surface 205A and the sidewall surface 205B of the first electrode 205E, and the top surface 201A of the dielectric layer 201. The resistance variable layer 209 has a resistivity (or conductivity) capable of switching between a high resistance state and a low resistance state, by application of an electrical voltage. In various embodiments, the resistance variable layer 209 includes at least one of dielectric materials comprising a high dielectric constant (high-k) dielectric material, a binary metal oxide or a transition metal oxide. In some embodiments, the resistance variable layer 209 includes nickel oxide, titanium oxide, hafnium oxide, zirconium oxide, zinc oxide, tungsten oxide, aluminum oxide, tantalum oxide, molybdenum oxide or copper oxide. Possible formation methods include pulse laser deposition (PLD) or ALD, such as ALD with a precursor containing zirconium and oxygen. In one example, the resistance variable layer 209 has a thickness in a range from about 10 angstrom to about 500 angstrom.

A second electrode material 211 is conformally deposited over the resistance variable layer 209. The second electrode material 211 may include suitable conductive material to electrically connect a subsequently formed resistance variable memory structure to other portions of an interconnect structure for electrical routing. The second electrode material 211 may comprise Pt, AlCu, TiN, Au, Ti, Ta, TaN, W, WN, Cu or combinations thereof. In at least one example, the second electrode material 211 has a thickness in a range from about 30 angstrom to about 3000 angstrom. In some embodiments, the first electrode material 205 and the second electrode material 211 have a same composition. In some embodiments, the first electrode material 205 and the second electrode material 211 have different compositions. Possible formation methods include electroless plating, sputtering, electro plating, PVD or ALD.

Referring back to FIG. 1, the method 100 continues with operation 104 in which a portion of the second electrode material and the resistance variable layer are etched to form a second electrode over a sidewall of the first electrode.

FIG. 2D is a cross-sectional view of the semiconductor structure 200 after performing operation 104. In at least one embodiment, a portion of the second electrode material 211 and the resistance variable layer 209 are anisotropically etched to form a spacer over the sidewall 205B of the first electrode 205E without lithography patterning process. The spacer includes a vertical portion 209B of a remaining resistance variable layer 209 over sidewall 205B of the first electrode 205E and a horizontal portion 209C of the remaining resistance variable layer 209 over the top surface 201A of the dielectric layer 201. The spacer further includes a second electrode 211E of the second electrode material 211 formed over the vertical portion 209B and the horizontal portion 209C of the remaining resistance variable layer 209. A resistance variable memory structure 230 including the first electrode 205E, the vertical portion 209B and the horizontal portion 209C of the resistance variable layer 209, and the second electrode 211E is formed.

In some examples, the semiconductor structure 200 further includes a cap layer 213 optionally formed between the remaining resistance variable layer 209 and the second electrode 211E, such as over the vertical portion 209B and the horizontal portion 209C of the resistance variable layer 209 and underlying the second electrode 211E as shown in FIG. 2E. In some embodiments, the cap layer 213 includes a conductive material that is capable of depriving oxygen from the resistance variable layer 209 and thus causing vacancy defects formed in the resistance variable layer 209. The cap layer 213 comprises titanium, tantalum or hafnium in some embodiments.

Referring back to FIG. 1, the method 100 optionally continues with operation 105 in which a conductive plug is formed contacting the second electrode.

FIG. 2F is a cross-sectional view of the semiconductor structure 200 after performing operation 105. An inter-level dielectric (ILD) layer 215 may be blanket formed over the resistance variable memory structure 230. A chemical mechanical polishing (CMP) process is further applied to the semiconductor structure 200 to planarize the ILD layer 215. The ILD layer 215 may include multiple dielectric layers. The ILD layer 215 may comprise silicon oxide, fluorinated silica glass (FSG), carbon doped silicon oxide, silicon nitride, silicon oxynitride, TEOS oxide, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), amorphous fluorinated carbon, low-k dielectric material, or combinations thereof.

An opening is etched in the ILD layer 215 to expose a portion of the second electrode 211E. A conductive material of a contact plug 217 may overfill the opening in the ILD layer 215. The conductive material may include copper, copper alloys, aluminum or tungsten. The possible formation methods include electroless plating, sputtering, electro plating or chemical vapor deposition (CVD). The excess conductive material outside of the opening is removed through a suitable process such as chemical mechanical polishing (CMP). The contact plug 217 having the conductive material is formed contacting the second electrode 211E of the resistance variable memory structure 230.

FIG. 2G is a planar view of the semiconductor structure 200. FIG. 2F is the cross-sectional view obtained from a vertical plane crossing line A-A′ in FIG. 2G. In FIG. 2G, the first electrode 205E is surrounded by the vertical portion 209B of the resistance variable layer 209. The vertical portion 209B of the resistance variable layer 209 is surrounded by the second electrode 211E. The vertical portion 209B of the resistance variable layer 209 and the second electrode 211E are closed loops surrounding the first electrode 205E. When the semiconductor structure 200 is cut through crossing line A-A′ in FIG. 2G, the second electrode 21 1E is illustrated as two portions on opposite sides of the first electrode 205E in FIG. 2F. The conductive structure 203 in FIG. 2F and the horizontal portion 209C of the resistance variable layer 209 in FIG. 2F are underlying the first electrode 205E and the second electrode 21 1E, respectively. Hence, the conductive structure 203 and the horizontal portion 209C in FIG. 2F are not shown in FIG. 2G.

FIG. 2H is a cross-sectional view of the semiconductor structure 200 having a resistance variable memory structure in various operations for data storage.

In a “forming” operation, a “forming” voltage is applied to the first and second electrodes 205E and 211E of the resistance variable memory structure 230. The “forming” voltage is high enough to generate a conductive portion in the vertical portion 209B of the resistance variable layer 209. In one example, the conductive portion includes one or more conductive filaments 250 to provide a conductive path such that the vertical portion 209B of the resistance variable layer 209 shows an “on” or low resistance state. The conductive path may be related to the lineup of the defect (e.g. oxygen) vacancies in the vertical portion 209B of the resistance variable layer 209. In some embodiments, the “forming” voltage is applied only one time. Once the conductive path is formed, the conductive path will remain present in the resistance variable layer 209B. Other operations may disconnect or reconnect the conductive path using smaller voltages or different voltages.

In a “reset” operation, a “reset” voltage high enough to break the conductive path in the resistance variable layer 209B is applied to the resistance variable memory structure 230 such that the resistance variable layer 209B shows an “off” or high resistance state.

In a “set” operation, a “set” voltage high enough to reconnect the conductive path in the resistance variable layer 209B is applied to the resistance variable memory structure 230 such that the resistance variable layer 209B shows the “on” or low resistance state. The “set” operation turns the resistance variable layer 209B to the low resistance state. By applying a specific voltage between two electrodes 205E and 211E, an electric resistance of the resistance variable layer 209B is altered after applying the specific voltage. The low and high resistances are utilized to indicate a digital signal “1” or “0”, thereby allowing for data storage.

FIG. 3 shows a cross-sectional view of a semiconductor structure 300 with the resistance variable memory structure 230 of FIG. 2D (or 2E) according to at least one embodiment of the present disclosure. The semiconductor structure 300 may be formed on a substrate 302 such as silicon, germanium, and/or a compound semiconductor material. The semiconductor structure 300 may include an access transistor that includes as a gate electrode 304, a drain region 306 and a source region 308 on opposite sides of the gate electrode 304. The gate electrode 304 is formed on a top surface of the substrate 302. The source region 308 and the drain region 306 are formed by implantation in a portion of the substrate 302. Multiple dielectric layers 310 are formed over the access transistor and the substrate 302. Details of the materials and fabrication methods of the multiple dielectric layers 310 can be found in the text associated with the dielectric layer 201 in the semiconductor structure 200 and are not repeated here.

Referring still to FIG. 3, a plurality of stacked metallization layers and via layers are formed over the access transistor and embedded in the dielectric layers 310. In at least one embodiment, the plurality of metallization layers includes four metal layers M1 to M4. Vertical columnar vias (V1 to V3, 203 and 217) interconnect the source region 232 and the drain region 231 to metallization layer M1, and connect different metallization layers M1, M2, M3, and M4. The plurality of stacked metal features and via layers provides interconnections between devices structures, circuits and/or inputs/outputs terminals. The metallization layers and via layers may include aluminum, aluminum alloy, copper, copper alloy, titanium, titanium nitride, tantalum, tantalum nitride, tungsten, metal silicide, or combinations thereof.

In some embodiments, there are at least three metallization layers overlying the access transistor and underlying the resistance variable memory structure 230. In this illustrated example, the resistance variable memory structure 230 is formed between metallization layers M3 and M4. Vertical columnar vias 203 and 217 electrically connect the resistance variable memory structure 230 to the metallization layers M3 and M4, respectively. The source region 308 of the access transistor is coupled to a source line SL in metallization layer M2 through columnar via V1, metallization layer M1 and columnar via V2. A word line WL is electrically coupled to the gate electrode 304 to provide a gate voltage to turn on the access transistor. The drain region 306 may be coupled to the first electrode 205E of the resistance variable memory structure 230 through columnar vias (V1 to V3 and 203) and metallization layers (M1 to M3). A bit line BL in metallization layer M4 is electrically coupled to the second electrode 211E of the resistance variable memory structure 230 through the columnar via 217.

Generally, some of the processes for forming the bottom metallization layers (M1 to M3) may have a process temperature higher than 400° C., such as the processes for annealing or dielectric layer formation. The stability of a resistance variable memory structure 230 may be affected by the high temperature processes if the resistance variable memory structure 230 is formed before the high temperature processes. In this embodiment, the resistance variable memory structure 230 is formed over the bottom metallization layers (M1 to M3). This disclosure eliminates the drawbacks of high temperature effect on the resistance variable memory structure 230 in bottom metallization layers (M1 to M3). Furthermore, there are several control lines (such as the source line, the word line and the bit line) used to control the operation of the resistance variable memory structure 230 and the access transistor. There are many spaces needed in bottom metallization layers for circuit routing to arrange these control lines. Advantageously, the resistance variable memory structure 230 is formed over the bottom metallization layers (M1 to M3). In this illustrated example, the resistance variable memory structure 230 is formed between metallization layers M3 and M4. In accordance with one or more embodiments of the present disclosure, there are more spaces for circuit routing by forming the resistance variable memory structure 230 in the upper metallization layers.

FIG. 4 shows a semiconductor structure 400 substantially similar to the semiconductor structure 300 disclosed in FIG. 3. The description of the common structures are not repeated here although fully applicable in the following embodiments as well.

The semiconductor structure 400 having two resistance variable memory structures 230A and 230B substantially similar to the resistance variable memory structure 230 of FIG. 2D (or 2E). Second electrodes 211E and 211S of the resistance variable memory structures 230A and 230B contact (or share) a same columnar via 217. Through the columnar via 217, a bit line BL in metallization layer M4 is electrically coupled to the second electrode 211E of the resistance variable memory structure 230A and the second electrode 211 S of the resistance variable memory structure 230B. Advantageously, the resistance variable memory structures 230A and 230B shared the same columnar via 217 provides extra space for functional integrated circuits and circuit routing.

The semiconductor structure 400 includes a first access transistor having a gate electrode 304A, a drain region 306A and a source region 308A on opposite sides of the gate electrode 304A. The semiconductor structure 400 also includes a second access transistor having a gate electrode 304B, a drain region 306B and a source region 308B on opposite sides of the gate electrode 304B. The first access transistor and the second access transistor are isolated by a shallow trench isolation (STI) structure 301 formed by dielectric materials. Multiple dielectric layers 310 are formed over the access transistors and the substrate 302.

Referring still to FIG. 4, a plurality of stacked metallization layers (M1 to M4) and via layers (V1 to V3, 203 and 217) are formed over the access transistors and embedded in the dielectric layers 310. The plurality of stacked metal features and via layers provides interconnections between the access transistors, the resistance variable memory structures (230A and 230B), circuits and/or inputs/outputs terminals.

During various operations of the resistance variable memory structure 230A, a source line SL1 in metallization layer M2 electrically coupled to the source region 308A of the first access transistor provides a source voltage. A bit line BL in metallization layer M4 electrically coupled to the drain region 306A of the first access transistor provides a drain voltage. A word line WL1 electrically coupled to the gate electrode 304A of the first access transistor is turned on with a gate voltage. The resistance variable memory structure 230A is able to perform previous mentioned “forming”, “set” and “reset” operations for data storage. During various operations of the resistance variable memory structure 230A, the gate electrode 304B of the second access transistor is turned off and the source region 308B is floating.

Likewise, the resistance variable memory structure 230B is able to perform previous mentioned “forming”, “set” and “reset” operations for data storage by applying specific voltages to the source region 308B (through a source line SL2 in metallization layer M2), the drain region 306B (through the same bit line BL in metallization layer M4) and the gate electrode 304B (through a word line WL2) of the second access transistor. During various operations of the resistance variable memory structure 230B, the gate electrode 304A of the first access transistor is turned off and the source region 308A is floating.

In some embodiments, the source line SL1 and the source line SL2 is a same source line. The resistance variable memory structures 230A and 230B share a same source line for the respective access transistors to provide extra space for functional integrated circuits and circuit routing. In certain embodiments, the source line SL1 and the source line SL2 have different source lines for circuit design concern.

In FIG. 4, the resistance variable memory structures (230A and 230B) and the shared columnar via 217 are formed overlying three metallization layers (M1 to M3). However, this disclosure is not limited to the above conditions for forming the resistance variable memory structures (230A and 230B) and the shared columnar via 217 above at least three metallization layers, and differing conditions that produce the above shared columnar via for at least two resistance variable memory structures are within the scope of this disclosure.

Various embodiments of the present disclosure may be used to improve the processes of a resistance variable memory structure. For example, the disclosed method 100 includes a single lithography patterning process in forming the first electrode 205E in operation 102. The second electrode 211E is formed by a spacer etching process without lithography patterning process in operation 104. The disclosed method 100 includes a single lithography patterning process (in operation 102) used to form both electrodes 205E and 211E. This disclosure eliminates drawbacks in other methods which use multiple lithography patterning process steps in patterning both the first and second electrodes. The manufacturing complexity and cost are reduced in accordance with some embodiments.

One aspect of the disclosure describes a semiconductor structure including a resistance variable memory structure. The semiconductor structure also includes at least three metallization layers over a substrate. A dielectric layer is over the at least three metallization layers. The resistance variable memory structure is over the dielectric layer. The resistance variable memory structure includes a first electrode disposed over the dielectric layer. The first electrode has a sidewall surface. A resistance variable layer has a first portion over the sidewall surface of the first electrode, and a second portion extending from the first portion away from the first electrode. A second electrode is over the resistance variable layer.

A further aspect of the disclosure describes a semiconductor structure including a resistance variable memory structure. The semiconductor structure also includes an access transistor. The access transistor includes a source region and a drain region on opposite sides of a gate electrode. At least three metallization layers are over the access transistor. The resistance variable memory structure is over the at least three metallization layers. The resistance variable memory structure includes a first electrode disposed over the conductive structure. The first electrode has a sidewall surface. A resistance variable layer has a vertical portion and a horizontal portion. The vertical portion surrounds the sidewall surface of the first electrode and the horizontal portion extends from the vertical portion away from the first electrode. A second electrode is over the resistance variable layer and surrounds the vertical portion of the resistance variable layer.

Another aspect of the disclosure describes a semiconductor structure including resistance variable memory structures. The semiconductor structure also includes a substrate. A first resistance variable memory structure adjacent to a second resistance variable memory structure disposed over the substrate. Each resistance variable memory structure includes a first electrode, a resistance variable layer and a second electrode. The first electrode is disposed over the substrate and has a sidewall surface. The resistance variable layer has a first portion over the sidewall surface of the first electrode, and a second portion extending from the first portion away from the first electrode. The second electrode is over the resistance variable layer. A contact plug contacts both second electrodes of the first and second resistance variable memory structures.

Although the embodiments and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. As one of ordinary skill in the art will readily appreciate from the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

What is claimed:
 1. A semiconductor structure, comprising: at least three metallization layers over a substrate; a dielectric layer over the at least three metallization layers; and a resistance variable memory structure over the dielectric layer, the resistance variable memory structure comprising: a first electrode disposed over the dielectric layer, the first electrode having a sidewall surface; a resistance variable layer having a first portion which is a closed loop and surrounds the sidewall surface of the first electrode and a second portion which extends from the first portion away from the first electrode; a second electrode disposed over the resistance variable layer; and a conductive cap layer disposed between the resistance variable layer and the second electrode a conductive plug contacting the second electrode and the resistance variable layer.
 2. The semiconductor structure of claim 1, wherein the conductive cap layer comprises titanium, tantalum or hafnium.
 3. The semiconductor structure of claim 1, further comprising a conductive structure embedded in the dielectric layer, and electrically connected to the first electrode and the at least three metallization layers.
 4. The semiconductor structure of claim 1, wherein each of the first electrode and the second electrode comprises at least one selected from the group consisting of Pt, AlCu, TiN, Au, Ti, Ta, TaN, W, WN, and Cu.
 5. The semiconductor structure of claim 1, wherein the resistance variable layer comprises a high-k dielectric material, a binary metal oxide or a transition metal oxide.
 6. The semiconductor structure of claim 1 further comprising an access transistor having a source region and a drain region underlying the at least three metallization layer, wherein the first electrode is electrically connected to the drain region of the access transistor.
 7. The semiconductor structure of claim 1, wherein the second electrode is a closed loop surrounding the resistance variable layer and the first electrode.
 8. The semiconductor structure of claim 1, wherein the second portion is a portion of the closed loop and surrounds the first electrode.
 9. The semiconductor structure of claim 1, wherein the second portion of the resistance variable layer is disposed between the second electrode and the dielectric layer.
 10. The semiconductor structure of claim 9, wherein the first portion is selectively configurable to form at least one conductive path between the first electrode and the second electrode.
 11. A semiconductor structure, comprising: an access transistor comprising a source region and a drain region on opposite sides of a gate electrode; at least three metallization layer over the access transistor; and a resistance variable memory structure over the at least three metallization layer, the resistance variable memory structure comprising: a first electrode disposed over the at least three metallization layer, wherein the first electrode is electrically connected to the drain region of the access transistor and has a sidewall surface; a resistance variable layer having a vertical portion and a horizontal portion, wherein the vertical portion is a closed loop and surrounds the sidewall surface of the first electrode and the horizontal portion extends from the vertical portion away from the first electrode; a second electrode disposed over the resistance variable layer and surrounding the vertical portion of the resistance variable layer; and a conductive cap layer disposed between the resistance variable layer and the second electrode a conductive plug contacting the second electrode and the resistance variable layer.
 12. The semiconductor structure of claim 11, wherein the vertical portion is selectively configurable to form at least one conductive path between the first electrode and the second electrode.
 13. The semiconductor structure of claim 11, wherein the resistance variable layer comprises a high-K dielectric material, a binary metal oxide or a transition metal oxide.
 14. The semiconductor structure of claim 11, wherein each of the first electrode and the second electrode comprises at least one selected from the group consisting of Pt, AlCu, TiN, Au, Ti, Ta, TaN, W, WN, and Cu.
 15. The semiconductor structure of claim 11, wherein the conductive cap layer surrounds the resistance variable layer.
 16. A semiconductor structure, comprising: a substrate; a first resistance variable memory structure adjacent to a second resistance variable memory structure disposed over the substrate, each of the first and the second resistance variable memory structure comprising: a first electrode disposed over the substrate and having a sidewall surface; a resistance variable layer having a first portion which is a closed loop and surrounds the sidewall surface of the first electrode and a second portion which extends from the first portion away from the first electrode; and a second electrode disposed over the resistance variable layer; and a conductive plug contacting both second electrodes and resistance variable layers of the first and second resistance variable memory structures.
 17. The semiconductor structure of claim 16 further comprising: a first access transistor and a second access transistor underlying the first and second resistance variable memory structures, wherein each of the first and second access transistors has a source region and a drain region, wherein the drain region of each access transistor is electrically connected to the first electrode of the first and second resistance variable memory structures respectively.
 18. The semiconductor structure of claim 17, wherein the source region of each access transistor is configurable to connect a same source line.
 19. The semiconductor structure of claim 16, wherein the first portion is selectively configurable to form at least one conductive path between the first electrode and the second electrode. 